1. Field of the Invention
The present invention relates to add/drop multiplexers and interfaces, and more particularly to an add/drop multiplexer and an interface which are suitable for connection between an existing digital signal network and a new synchronous network called SONET (Synchronous Optical NETwork).
2. Description of Related Art
The existing transmission networks in North America or other areas are a digital signal network (hereinafter called asynchronous DSn network or DSn network), in which signal transfer takes place in terms of digital signals (DSn; n=1, 2, 3, . . . ) such as DS1 (Digital Signal level 1; 1.544 Mbps), DS2 (6.132 Mbps), DS3 (44.736 Mbps), and a new synchronous network called SONET (Synchronous Optical NETwork), in which signal transfer takes place in terms of synchronous transport signals (STSm; m=1, 3, 12, 48, 192 . . . ) such as STS1 (Synchronous Transport Signal level 1; 51.84 Mbps), STS3 (155.52 Mbps), STS12 (622.08 Mbps).
DS2 signals correspond to the signals on which DS1 signals are mapped by four channels, and DS3 signals correspond to the signals on which DS2 signals are mapped by seven channels. Likewise, STS3 signals correspond to the signal in which STS1 signals are mapped by three channels, STS12 signals correspond to the signals on which STS3 signals are mapped by four channels. Thus in SONET, signal transfer takes place always in terms of STS1 signals, though they are different in number of mapping channels of low-order-group signals.
STSm is a name in electrical signal level, and is alternatively called OCm (Optical Carrier level m) in optical signal level.
As well known in the art, for interconnecting DSn network and SONET, an add/drop multiplexer (ADM) (hereinafter also called ADM node or simply node) are used. For example, as depicted in FIG. 17 of the accompanying drawings, nodes 101, 102 are put into practice which connect asynchronous DS3 network 100 for transfer of DS3 signals and SONET 200 for signal transfer in terms of STS1 signals, in which DS3 signals are mapped over the payload region (SPE: Synchronous Payload Envelope). Namely, as schematically depicted in FIG. 19(A), nodes 101, 102 having a function of mapping DS3 signals on STS1 signals and demapping DS3 signals off DS3-mapped STS1 signals are put into reality.
Now assume that SONET 200 supports services of transfer of VT1.5 signals in terms of STS1 signals. By providing the ADM node 101 or 102 with a function of signal conversion function between DS3 signals and VT1.5 signals and a function of signal mapping/demapping between VT1.5 signals and STS1 signals, it is possible to add DS3 signals, which are to be transferred in DS3 network 100, to SONET 200 by mapping DS3 signals on STS1 signals in terms of VT1.5 signals, and also to drop signals, which are being transferred in SONET 200 as mapped on STS1 signals in terms of VT1.5 signals, to the DS3 network 100 by demapping the last-named signals as VT1.5 signals.
An exemplary arrangement of connection interface of the ADM node 101 (102) is shown in FIG. 18. The node 101 (102) comprises a B/U (Bipolar/Unipolar) converter 111, a DS3/DS2-demapping converter 112, a DS2/DS1-demapping converter 113, a DS1-format-conversion memory 114, a DS1/VT1.5 mapping (MAP) section 115, an STS1 mapping (MAP) section 116, an STS1-network clock generator 117, a SONET-condition notification section 118, an STS1 demapping (DMAP) section 121, a VT1.5/DS1 demapping (DMAP) section 122, a SONET-destuff memory 123, a DS1/DS2-demapping converter 124, a DS2/DS3-demapping converter 125, a U/B (Unipolar/Bipolar) converter 126, a DS1 clock generator 127, and a DS3-network clock generator 128.
The B/U converter 111 converts outside-apparatus bipolar signals (DS3 signals), which are received from the DS3 network 100, into in-apparatus unipolar signals. The DS3/DS2-demapping converter 112 converts the mapped DS3 signals into demapped DS2 signals. The DS2/DS1-demapping converter 113 converts the DS2 signals, which have been obtained by the DS3/DS2-demapping converter 112, into further demapped DS1 signals.
The DS3/DS2-demapping converter 112 and the DS2/DS1-demapping converter 113 respectively detect high-order-group alarms (AIS: alarm indication Signal) with respect to before-demapping-converted high-order-group signals by a DS3 alarm detector 112a and a DS2 alarm detector 113a. Upon detection of a high-order-group alarm by the alarm detector 112a or 113a, the converters 112 and 113 send such message to the SONET condition notification section 118, and output signal (DS1-AIS inserting signal (command)), for inserting AIS in terms of DS1 signal during subsequent mapping (format-converting) of DS1 signals to VT1.5 signals, to the DS1/VT1.5 mapping section 115.
Further, the DS1-format-conversion memory 114 stores the DS1 signals, which have been obtained by the DS2/DS1-demapping converter 113, into a VT1.5-signal format, and the DS1/VT1.5 mapping section 115 reads out from the DS1 signals, which have been stored in the DS1-format-conversion memory 114, in synchronism with clocks, which are generated by the STS1-network clock generator 117, and converts the read-out signals into a VT1.5-signal format.
Furthermore, VT1.5 signals input from the DS1/VT1.5 mapping section 115 are mapped over STS1 signals at VT1.5 mapping section 116a in synchronism with clocks, which are generated by the STS1-network clock generator 117. And the STS1 mapping section 116 outputs the resulting signals to SONET 200 as the VT1.5-mapped STS1 signals.
At that time, if it receives a DS1-AIS inserting signal from the DS3 alarm detector 112a or the DS2 alarm detector 113a, the STS1 mapping section 116 sets all data of the DS1 signals, which are mapped over the DS3 signals or DS2 signals, to “1” (inserts DS1-AIS) and outputs the resulting signals as STS1 signals.
And the SONET-condition notification section 118 serves to notify a network administrator or the like, via a non-illustrated in-apparatus-condition monitor, of the message that an alarm has been detected by the DS3 alarm detector 112a or the DS2 alarm detector 113a. 
In the meantime, the STS1 demapping section 121 terminates STS1 signals, which have received from SONET (network supporting transfer of VT1.5 signals) 200, and demapps the terminated STS1 signals in terms of VT1.5 signals by the VT1.5 demapping sub-section 121a. And the VT1.5/DS1 demapping section 122 terminates the VT1.5 signals, which have been obtained by the STS1 demapping section 121 (VT1.5 demapping sub-section 121a), and maps (format-conversion) the terminated VT1.5 signals to DS1 signals.
The STS1 demapping section 121 and the VT1.5/DS1 demapping section 122 respectively detect high-order-group alarms (STS1 alarms with respect to VT1.5 signals, DS1 signals) by an STS alarm detector 121b and a VT alarm detector 122a. Upon detection of a high-order-group alarm by the STS alarm detector 121b or the VT alarm detector 122a, the STS1 demapping section 121 and the VT1.5/DS1 demapping section 122 respectively send DS1-AIS inserting signals to the DS1/DS2-mapping converter 124.
In order to equalizing DS1 signals by absorbing possible stuff of data, which has occurred when destuffing STS1 signals and VT1.5 signals contained in DS1 signals, the SONET-destuff memory 123 stores DS1 signals, which are input from the VT1.5/DS1 demapping section 122, and outputs the input DS1 signals in synchronism with clocks, which are generated by the DS1 clock generator 127. And the DS1/DS2-demapping converter 124 maps the DS1 signals, which have been equalized by the SONET-destuff memory 123, to DS2 signals in synchronism with clocks, which are generated by the DS3-network clock generator 128.
At that time, upon receipt of a DS1-AIS inserting signal from the STS alarm detector 121b or the VT alarm detector 122a, the DS1/DS2-demapping converter 124 sets all data of the DS1 signals, which are mapped over the STS1 signals or DS2 signals, to “1” (inserts DS1-AIS).
And the DS2/DS3-mapping converter 125 converts DS2 signals, which are input from the DS1/DS2-mapping converter 124, into DS3 signals in synchronism with clocks, which are generated by the DS3-network clock generator 128. The U/B converter 126 converts DS3 signals (unipolar signals), which have been obtained by the DS2/DS3-mapping converter 125, into bipolar signals and outputs these resulting signals to the DS3 network 100.
With the foregoing arrangement of the conventional node 101 (102), STS1 signals (STS1-mapped VT1.5), over which VT1.5 signals to be received from SONET 200 have been mapped, are terminated by the STS1 demapping section 121. At that time, upon detection of a high-order-group alarm by the STS alarm detector 121b, a DS1-AIS inserting signal is notified to the DS1/DS2-mapping converter 124.
The STS1 signals terminated by the STS1 demapping section 121 are then demapped in terms of VT1.5 signals by the VT1.5 demapping sub-section 121a, and the resulting signals are output to the VT1.5/DS1 demapping section 122, where VT1.5 signals input from the STS1 demapping section 121 (VT1.5 demapping sub-section 121a) are terminated. At that time, upon detection of a high-order-group alarm by the VT alarm detector 122a, a DS1-AIS inserting signal is notified to the DS1/DS2-mapping converter 124.
Then the terminated VT1.5 signals are input to the SONET-destuff memory 123 after demapped into DS1 signals, whereupon the memory 123 equalizes DS1 signals by absorbing possible stuff of STS1 signals and VT1.5 signals contained in input DS1 signals and are then output to the DS1/DS2-mapping converter 124. Then DS1/DS2-mapping converter 124 converts the equalized DS1 signals into DS2 signals in synchronism with clocks, which are generated by the DS3-network clock generator 128, and outputs the resulting signals to the DS2/DS3-mapping converter 125.
At that time, if received a DS1-AIS inserting signal from the STS alarm detector 121b or the VT alarm detector 122a, the DS1/DS2-mapping converter 124 sets all data of DS1 signals, which are contained in STS1 signals or VT1.5 signals by the DS1-AIS inserting section 124a, to “1” to output STS1 signals.
Then the obtained DS2 signals are converted into DS3 signals by the DS2/DS3-mapping converter 125, and the converted signals are further converted into unipolar signals by the U/B converter 126, whereupon the resulting signals are sent to the DS3 network 100.
In the meantime, DS3 signals received from the asynchronous DS3 network 100 are converted into unipolar signals by the B/U converter 111, and the converted signals are then terminated by the DS3/DS2-demapping converter 112, whereupon the terminated signals are demapped to DS2 signals. During terminating of DS3 signals, upon detection of a high-order-group alarm (DS3 alarm) by the DS3 alarm detector 112a, such message is notified to the SONET-condition notification section 118 and, at the same time, a DS1-AIS inserting signal is sent to the DS1/VT1.5 mapping section 115.
The DS2 signals thus obtained by the DS3/DS2-demapping converter 112 are then terminated by the DS2/DS1-demapping converter 113, whereupon the terminated signals are demapped to DS1 signals. Also in this case, if a high-order-group alarm (DS2 alarm) has been detected by the DS2 alarm detector 113a during termination of the DS2 signals, the DS3/DS2-demapping converter 112 sends such message to the SONET-condition notification section 118 and, at the same time, sends a DS1-AIS inserting signal to the DS1/VT1.5 mapping section 115.
The DS1 signals obtained by the DS2/DS1-demapping converter 113 are then stored in the DS1-format-conversion memory 114 and are read out in synchronism with clocks generated by the STS1-network clock generator 117, whereupon the DS1 signals are mapped (format-conversion) to VT1.5 signals by the DS1/VT1.5 mapping section 115. At that time, if the DS1-AIS inserting signal was received from the DS3 alarm detector 112a or the DS2 alarm detector 113a as mentioned above, the DS1-VT1.5 mapping section 115 sets (inserts DS1-AIS) all the data of the DS1 signals, which are mapped on STS1 signals or DS2 signals by DS1-AIS inserting sub-section 115a, to “1” and then outputs the resulting STS1 signals to the STS1 mapping sub-section 116a. 
In the STS1 mapping section 116, the VT1.5 mapping sub-section 116a maps VT1.5 signals, which are from the DS1/VT1.5 mapping section 115, on VT1.5 signals and sends the resulting signals to SONET 200.
The foregoing conventional node 101, 102 realizes signal conversion between DS3 signals and STS1 signals, as schematically depicted in FIG. 19(B). Therefore, as shown in FIG. 17, signal add/drop (cross-connect) can be realized in terms of VT1.5 signals between the DS3 network 100, on which DS3 signals are transferred, and the new synchronous network (SONET) 200, on which signals are transferred in terms of STS1 signals.
However, according to the conventional node 101 (102), because DS3 signals cannot be mapped on STS1 signals to be transferred to SONET 200, it is impossible to realize signal transfer in terms of DS3-mapped STS1 signals in SONET 200 so that services in terms of DS3 signals cannot be provided. Further, because STS1 signals already mapped on DS3 signals in SONET 200 cannot be mapped/demapped (restructured) in terms of VT1.5 signals, it is impossible to provide services, such as signal add/drop (cross-connect), DS3-mapped STS1 signals only in terms of STS1 signals. The foregoing description can be said also in case of SDH (Synchronous Digital Hierarchy) according to TTC (Telecommunication Technology Committee) standards.